Plane wave interfering sources have long posed problems to our sonar signal processors' performance. As sonar systems are improved and detection ranges become longer, this increased sensitivity to targets also implies an increased sensitivity to interference. Since both are plane waves which propagate across the array, and since the basic purpose of beamforming is to discriminate against the spatially incoherent noise and enhance coherent plane waves, it might at first seem to be a rather hopeless situation. However, the supposition in beamforming, as in all signal processing, is simply to discriminate against signal and noise based upon the differences between their spatial and temporal characteristics. It is possible, in general, to discriminate against signal noise (or as the case may be, to discriminate against signal-plus-noise and noise only) based upon spectral, waveform, statistical, and spatial organization differences. The interference problem may be treated in any or all of these categories.